Synthesis and investigation of physico-chemical properties of dicationic ionic liquids

A series of dicationic ionic liquids (ILs) including [C 4 (MIM) 2 ][PF 6 ] 2 , [C 5 (MIM) 2 ][PF 6 ] 2 , [C 6 (MIM) 2 ][PF 6 ] 2 and [C 4 (PYR) 2 ][PF 6 ] 2 were synthesized. Their thermal stability and melting points were analysed. It was found that dicationic ILs presented important implications in the design of homogeneous and heterogeneous system with water. A homogeneous system of dicationic ILs with water could be formed at a relatively high temperature and then a heterogeneous system was formed when the solution was cooled to a low temperature. The ILs recovered by altering the temperature were obtained in high percentage yields of [C 4 (MIM) 2 ][PF 6 ] 2 (97.6%), [C 5 (MIM) 2 ][PF 6 ] 2 (97.3%), [C 6 (MIM) 2 ][PF 6 ] 2 (98.0%) and [C 4 (PYR) 2 ][PF 6 ] 2 (94.2%). On the other hand, [C 4 (MIM) 2 ][PF 6 ] 2 and [C 5 (MIM) 2 ][PF 6 ] 2 exhibited good solubility in acetonitrile and acetone. A homogeneous system could be achieved with imidazolium-based ILs with a relatively low amount of water and acetonitrile at room temperature. All of the properties of dicationic ILs have a strong correlation with the nature of dications, the linkage chain and the symmetry of dications. Dicationic ILs may provide a new opportunity for some specific applications in order to enable the effective separation and isolation of products.